Method of spray drying phenol-formaldehyde resin compositions

ABSTRACT

A pre-cure resistant powder resin binder for waferboard/oriented strandboard manufacture is manufactured from spray drying a liquid resin composition containing a low advanced phenol-formaldehyde resin and a water soluble oxo compound of boron. The spray dryability of the low molecular weight phenol-formaldehyde resin is enhanced by the addition of the oxo boron compound.

BACKGROUND OF THE INVENTION

This invention relates to the formulation and manufacture of a spray dryresole type phenol-aldehyde resin containing water soluble oxo compoundsof boron. The powdered resin has pre-cure resistant property and findsparticular utility for making composition boards such as that ofwaferboard and oriented strandboard (OSB) manufacture.

At present, most Canadian waferboards and oriented strandboards aremanufactured with powdered phenol-formaldehyde resins. The powderedresins have a number of advantages over liquid resins, such as: (a)simple and less expensive equipment can be used for powder resindelivery and blending with wood furnish; (b) powdered resins are easy toblend onto wood wafers and strands, achieving uniform resindistribution; and (c) powdered resins have long storage lives.

In industrial practice, resins with different properties are appliedseparately onto face and core furnish of waferboard/orientedstrandboard. The face resin blended wood furnish is used for facelayers, and the cored resin blended wood furnish is for core layerconstruction of waferboard/oriented strandboard. The face resin requirespre-cure resistance, non-sticking to platen and light color after cure.The core resin requires the ability to cure fast. The two resin (faceand core) system has the advantages of producing high quality board andincreasing mill productivity by reducing hot pressing times.

In earlier industrial practice, novolac type phenol-formaldehyde powderresins were used for waferboard/oriented strandboard manufacture. Thenovolac resins have excellent pre-cure resistant properties due toexcellent thermal flow under heat and pressure, and they also have alight color on the wood board surface after curling. However, thenovolac resins have the disadvantages of sticking on caul plate andplaten surfaces and being of considerably higher cost to manufacturethan spray dried resole resins.

For waferboard/oriented strandboard manufacture, the spray dried resoleresins require a property of pre-cure resistance for face layerapplications. To have the pre-cure property, the powder resin must havea significantly longer thermal flow property under heat and pressurethan that of the core resin. The thermal flow property is mainlycontrolled by the molecular weight of the resin. Low molecular weightresins show higher thermal flow property and better pre-cure resistancethan high molecular weight resin. However, spray drying a resole resinhaving the desired pre-cure resistance is difficult due to poor spraydryability of the low moleuclar weight resin.

In the spray drying process, a liquid phenol-aldehyde resin is atomizedto fine droplets and mixed with hot air (180° to 210° C.) to evaporatethe water from the resin droplets. The temperature of the resin isusually close to the boiling water temperature or higher. Afterevaporation of water, the low molecular weight phenol-aldehyde resinparticles are soft and tacky. The low softening particles tend todeposit on the surfaces of the spray dryer chamber and cyclone. Theundesired deposit of resin is severely detrimental to the spray dryingoperation by reducing powder resin yield, increasing fire hazard,frequently shutting down operation for cleaning, and undesirablyincreasing powder particle size. Therefore, the spray drying of lowmolecular weight resole has the problems of low powder resin yield, highcost of production, low spray dryer productivity, and the production ofinferior products.

DESCRIPTION OF THE PRIOR ART

Spray drying phenol-aldehyde resins is a known technology. U.S. Pat. No.4,098,770 (Berchem et al) described that spray dried powder resin wasprepared by spray drying a highly-advanced resole. The patent alsosuggested that a non-phenolic polyhydroxy compound may be mixed with theliquid resin before spray drying. The non-phenolic polyhydroxy compoundswere propylene glycol, butylene glycol, and ethylene glycol, which areplasticizers for powdered resins. However the patent suggested neitherto spray dry a low molecular weight phenol-aldehyde resin nor a solutionto the problems of spray drying the resin.

U.S. Pat. No. 4,424,300 (Udvardy et al) indicated that a powdered resincomposition was made by spray drying a homogeneous liquid resin mixturewhich contained a novolac resin and a resole resin. The novolac andresole resins were made by conventional methods. The resins were mixedbefore spray drying. This patent states that the powder resin had afaster cure property than novolac resin and better thermal flow propertythan residue powder resin.

Boric acid is known to be a catalyst for making high ortho-linkagenovolac resins. After compounding with hexamethylenetetramine, the boricacid catalyzed resin cured faster than the conventional acids catalyzednovolac resins.

Furthermore, boric acid and borax were suggested as fire retardantagents for phenol-formaldehyde resin foam and phenolic plasticlaminations.

U.S. Pat. No. 3,298,973 (Quarles et al) indicated that a fire resistantphenol-formaldehyde resin foam was made by catalyzing a mixture ofresole resin and foaming agent with boric acid and an organic acid.Chemical Abstract: CA88(6):38604 (Ger. Offen. DE 2717775) showed thatborax was used in phenol-formaldehyde resin foams as fire retardant andto reduce acidic corrosion of metals in contact with the foam.

U.K. Patent 901,663 (Lowe et al) indicated that fire resistant laminatedplastics were comprised of fibrous materials and a resole resin in whichthe alkaline catalyst was neutralized with boric acid.

SUMMARY OF THE INVENTION

This invention relates to the composition and manufacture of a spraydried powder resin containing a low advanced resole resin. Specifically,a relatively low molecular weight liquid phenol-aldehyde resin isformulated, and manufactured, and then an oxo compound of boron is mixedinto the resin prior to the spray drying process. Spray dryability ofthe low advanced resin is enhanced by the addition of the oxo boroncompound.

The oxo boron compound is believed to form chelate complexes with thephenol-aldehyde resin molecules. The complex formation improves thespray dryability of the resin and reduces advancement, i.e., increase ofmolecular weight, of the resin during the spray drying process.Therefore, the spray dried resins have a low molecular weight andpre-cure resistance particularly that needed for waferboard/orientedstrandboard manufacture.

The powder resin manufacture involves the following steps: (a)preparation of low molecular weight resole with phenol, aldehyde, and analkaline catalyst; (b)mixing the boron compound with the resin such asthe preparation of an aqueous solution of oxo boron compound and mixingthe resin with the boron compound solution; and (c) spray drying theresin composition. Advantages of the invention include:

1. Spray dryability of low advanced phenol-aldehyde resin issubstantially improved. During spray drying, little powder resindeposits on the wall of the dryer chamber and cyclone. Therefore, aconsistent quality of powder resin is produced, powder resin yield isincreased, and fire hazard in the spray dryer is reduced.

2. The spray dried resins have the desired thermal flow property andpre-cure resistance for waferboard/oriented strandboard manufacture.

3. Productivity of the spray dryer is improved by reducing plant shutdown for cleaning.

4. Reduces oxidation of the phenol-aldehyde resin; therefore, a tancolor formation of the powder resin during storage is reduced.

5. The powder resins have high thermal flow property and may also beused as molding compounds.

6. Waferboard and oriented strandboard made with the powdered resins ofthis invention have improved properties such as higher internal bondstrength.

DETAILED DESCRIPTION

Three steps are involved in preparing the powdered resins of thisinvention. These steps are: (a) preparation of a low advancedphenol-aldehyde resin; (b) addition or mixing of a water soluble oxoboron compound into the aqueous resin; and (c) spray drying of theliquid resin containing the boron compound to form the powdered resin.

A. Preparation of the Phenol-Aldehyde Resin

The phenol-aldehyde low molecular weight resole resins of this inventionare preferably made from phenol and formaldehyde with sodium hydroxideas the catalyst. The phenol may be partly substituted with xylenols,cresols, catechol, and the naturally occurring alkyl phenols, such ascresylic acids. The aldehyde can be formaldehyde or other aldehydes asis well known in the phenolic resole arts such as paraformaldehyde,acetaldehyde, and propionaldehyde. Sodium hydroxide is preferably usedas the catalyst for formation of the resin, although other alkalinecatalysts conventionally used to catalyze the formation of resoles suchas other alkali metal hydroxides, allkaline earth hydroxides, and metalcarbonates such as sodium or potassium carbonate may be used. Theaqueous resole resin is preferably made in a two-stage process. For thefirst stage reaction in forming the resin, the molar ratios ofphenol:aldehyde:alkaline catalyst are preferably 1:(1.4-1.6):(0.1-.15)and, more broadly, 1:(1.2-1.8):(0.05-0.25). The first stage reactiontemperature is from about 90° C. to reflux. The final molar ratios ofphenol : aldehyde : alkaline catalyst are preferably1:(1.8-2.2):(0.1-0.15) and, more broadly, 1:(1.5-2.5):(0.05-0.25). Thedesired resin molecular weight distribution is preferably manipulated bya two-stage aldehyde addition and two-level reaction temperatures. Thefirst stage involves a higher temperature reaction which is favorablefor the condensation reaction to build higher molecular weight resin.The second stage is a low temperature reaction which is favorable forthe methylolation type of reaction between phenol and aldehyde.

The solid contents in the aqueous solution are 50 to 60% at the end ofthe first stage. The resin is condensed to a viscosity (25° C.) ofGardner Holdt preferably E to G, which is about 1100 to 1700 weightaverage molecular weight (Mw) by the gel permeation chromatographymethod for this first stage. At the end if the first stage, the resincontains residual free phenol and higher molecular weight resin. Thereaction temperature is then cooled to 65° to 70° C. for the secondstage reaction.

For the second stage reaction, the second portion of formaldehyde isadded to the first stage reacted resin. The final molar ratios offormaldehyde to phenol are preferably 1.8 to 2.2:1. The second stage ispreferably reacted at 60° to 70° C. until the free formaldehyde contentis below 1 to 3% based on the liquid resin. The second stage reactiontemperature can vary from about 55° C. to 70° C. Owing to low reactiontemperatures, the second stage is mainly a methylolation reaction andthe condensation reactions to form higher molecular weight resins areminimized.

It is preferable that an anionic surfactant at 0.2 to 0.4% based on theliquid resin is added to reduce surface tension of the liquid resin inorder to obtain a desired fine particle size of the spray dried powderresin.

Advantageously, ammonia water, (such as that of 28 to 30% concentration)of 1.5 to 2.5% based on the liquid resin, is added to the resin to reactwith the residual free formaldehyde to form stable nitrogen compounds inthe resin. The nitrogen compounds will contribute a desired yellowishcolor of the resin spots is close to the natural wood color of thewaferboard/strandboard. The ammonia water may b*e substituted partiallyor completely with another nitrogen containing compound such ashexamethylenetetramine, dicyandiamide and melamine.

The resin, prior to spray drying, preferably contains 35 to 45% of lowmolecular weight resin based on the resin solids by weight ofmethlolated phenols and di-nuclear and tri-nuclear phenols. The range ofthe weight average molecular weight for this fraction is about 120 to520 determined by gel permeation chromatography. This fraction of theresin significantly contributes to the thermal flow property of thespray dried resin. The overall weight average molecular weight for theliquid resin before spray drying is from about 1200 to 1800.

The liquid resins used in this invention will have a final viscosity ofabout 200 to 900 Cps at 25° C. and preferably 250 to 500 Cps at 25° C.and a resin solids content of about 40% to 50% based on the aqueousresin solution or dispersion before addition of the water soluble oxoboron compound. The term "resin solids" as used herein refers tonon-volatile content of the liquid resin is determined by Test Method2.2 of the West Coast Adhesives Manufacturers.

Depending on waferboard/oriented strandboard manufacturing conditions,the content of the low molecular weight resin fraction can be adjustedby the liquid resin manufacture technique. For example, to keep thefinal aldehyde to phenol molar ratio constant, the low molecular weightresin fraction can be increased by reducing the aldehyde to phenol molarratio of the first stage reaction. Whereas, aldehyde to phenol molarratios of the first stage reaction can be increased to reduce thecontent of the low molecular weight fraction.

B. Modification of the Phenol-Aldehyde Resin with Water Soluble OxoCompounds of Boron

The oxo boron compounds dissolve very slowly in the phenol-aldehyderesin at ambient temperature. Therefore, it is preferred that an aqueoussolution of boron compound is first prepared and then mixed with theresin. Alternatively, the oxo boron compounds may be added to the resinduring the second stage cook. Increasing the temperature of the resincan improve the solubility

It requires only a small amount of the boron compound to significantlyimprove spray dryability of the resole resins and retard increase ofmolecular weight of the resin. The improvement of the spray dryabilityis initially proportional to the quantity of boron compound in the resinand then levels off. The quantity of boron compound in the resin forimproving the spray drying and retarding increase of molecular weightcan vary over a wide range such as that of about 0.05% to 5% or more ofthe boron compound on an anhydrous basis in relation to resin solids ofthe resole resin. The preferred contents of the boron compound on ananhydrous basis is from about 0.1% to 2% and particularly from about0.5% to 1.5% based on the resin solids.

Any boron compound having the requisity solubility in water is suitablefor use in this invention. The water soluble boron compounds are oxygencontaining boron compounds, also referred to herein as oxo boroncompounds. Additionally, insoluble boron compounds which are convertedto oxygen-containing water soluble compounds in the phenol-aldehyderesin are also operable. Illustrative of suitable boron compounds therecan be mentioned boric acid, metaboric acid, and the various borates andmetaboric acid salts such as that of borax decahydrate (Na₂ B₄ O₇.10H₂O); borax pentahydrate (Na₂ B₄ O₇.5H₂ O); potassium borates; lithiumborates; ammonium borates; sodium metaborates; potassium metaborates;and ammonium metaborates. Boric acid and sodium borate are the preferredwater soluble oxo compounds of boron.

C. The Spray Drying Process

In spray drying, the liquid feed of the boron containing phenol-aldehyderesin is converted to a fine spray; the water in the feed is evaporatedby means of a stream of hot air; and the dry, powdered product isseparated from the stream of hot air. Moisture evaporation is controlledby control of the inlet and outlet temperature of the hot air used fordrying. Due to the thermosetting nature of the product, the inlettemperature of the hot air is generally from about 180° C. to 210° C.and preferably from almost 185° C. to 195° C. The outlet temperature isgenerally from about 60° C. to 95° C. and preferably from about 75° C.to 90° C. Preferably, the phenol-aldehyde resin supplied to the spraydryer will have a resin solids content of about 25% to 45% by weight ofthe aqueous resin solution and a viscosity of about 30 to 300 Cps at 25°C. This differs from the resin as prepared in the above section entitled"Preparation of Phenol-Aldehyde Resin" and is due primarily to theaddition of water with the boron compound or simply the addition ofwater to dilute the resin for spray drying.

The particle size distribution, moisture content, and thermal flow ofthe spray dried resins are controlled by operations well known in thespray drying art by variables such as infeed resin solids content andsurface tensions, speed of the rotary atomizer, feed rate, and thetemperature differences of the inlet and outlet. Particle sizedistribution is an important factor in production of a powdered resin.This is particularly the case for waferboard/oriented strandboardapplications. At a given resin add-on level, the smaller the powderresin particle size, the more areas of wood wafers or strands arecovered with the resin. The preferred particle sizes for thewaferboard/oriented strandboard applications are that 80 to 90% of thepowder resin is less than 75 microns and 60 to 70% is less than 45microns.

Moisture content of the spray dried resin affects the free flow propertyof the powder. Owing to the hygroscopic property of phenol-formaldehyderesin, the highly moist product tends to cake during storage. Therefore,the moisture contents should preferably be below 2 to 3%. To reduce thecaking problem, drying agents, preferably calcium silicate and/or limeat 0.5 to 2.0% based on powder resin weight, may be mixed with the resinbefore bagging.

Thermal flow property of the powder resin depends mostly on themolecular weight of the resin. During the spray drying process, heat mayincrease the molecular weight. Therefore, the spray dryer should beoperated at as low a temperature as possible.

The weight average molecular weight of the spray dried resin will varyfrom about 1250 to 1900 and preferably about 1300 to 1700.

D. Manufacture of the Boards

The waferboard or strandboard is manufactured by conventional techniqueswith use of the powdered resins of this invention instead of the priorart powdered phenol-aldehyde resins. Thus, heat and pressure are appliedto a mat of wafers or oriented strands of wood coated with theboron-containing powdered resin of this invention in order to cure theresin and form the board. Conventional temperatures, time periods,pressures, and quantity of resin binder are used. Thus, the temperaturecan vary from about 195° C. to 220° C., the pressure can vary from about450 to 700 psi, and the quantity of powdered resin coating the wafers orstrands can vary from about 1.5 to 5% based on the weight of the wafersor strands.

The following examples illustrate the invention.

EXAMPLE 1

This example shows the preparation of a low advanced phenol-formaldehyderesin and the improvement of the spray dryability of the resin bytreating the liquid resin with borax pentahydrate prior to spray drying.The powder resin made from spray drying the resin is pre-cure resistantfor waferboard/oriented strandboard manufacture.

A resin reactor was charged with the following ingredients:

    ______________________________________                                                              Parts by   Molar                                        Ingredients           Weight     Ratio                                        ______________________________________                                        Phenol (92.0% conc.)  33.83      1                                            First Fromaldehyde (46.5% conc.)                                                                    32.04      1.5                                          First Water           10.48                                                   Sodium Hydroxide (50% conc.)                                                                        3.48       0.13                                                               (79.83)                                                 Second Fromaldehyde (46.5% conc.)                                                                   12.81      0.6                                          Second Water          2.75                                                    Third Formaldehyde (46.5% conc.)                                                                    2.10       0.1                                          Ammonium Hydroxide (28% conc.)                                                                      2.19                                                    Oleic Acid            0.32                                                                          (20.17)                                                 ______________________________________                                    

The reactor was charged with phenol, first formaldehyde, and firstwater. Then the sodium hydroxide was added slowly over a 10 to 15 minuteperiod and the temperature was allowed to rise to 95° C. in 50 minutes.The resin was reacted at 95° C. to a Gardner-Holdt viscosity (25° C.) ofE to F, then cooled to 75° C. to 70° C. over 10 to 15 minutes. Then thesecond formaldehyde and the second water were added and the temperaturewas adjusted to 70° C. The resin was reacted at 70° C. to a viscosity(25° C.) of G to H, then cooled to 55° to 50° C. The third formaldehydeand ammonium hydroxide solution were added. The reaction was continuedat 60° C. to a viscosity (25° C.) of L to M. Then the resin was cooledto below 25° C. The resin properties were as follows:

    ______________________________________                                        Viscosity (25° C.)                                                                        LL-M (Gardner-Holdt)                                       Solids Content     47.0%                                                      Refractive Index (25° C.)                                                                 1.4683                                                     Water Dilutability (25° C.)                                                               198%                                                       ______________________________________                                    

The resin was further treated with borax pentahydrate prior to spraydrying as follows:

    ______________________________________                                                    Parts By Weight                                                   Ingredient    Control    Resin A  Resin B                                     ______________________________________                                        Borax Pentahydrate                                                                          0          0.2      0.3                                         (Na.sub.2 B.sub.7 O.sub.4.5H.sub.2 O)                                         Water         36.1       36.1     36.4                                        Liquid Resin  63.9       63.7     63.3                                        (47.0% solids)                                                                ______________________________________                                    

The Resin A and Resin B were prepared by dissolving the boraxpentahydrate in the water and then mixing with the liquid resin. Thecontrol was the resin sample without the borax treatment. The propertiesof the resins were as follows:

    ______________________________________                                                        Control                                                                              Resin A  Resin B                                       ______________________________________                                        Resin Solids Content                                                                            30.0%    30.1%    30.1%                                     Refractive Index (25° C.)                                                                1.4139   1.4144   1.4163                                    Viscosity (25° C.)                                                                       A1       A1       A1-AA                                     (Gardner-Holdt)                                                               ______________________________________                                    

The effect of the borax treatments on the resin spray dryability wasevaluated by comparing the spray dryability of Resin A and Resin B withthe Control. The resins were spray dried with a laboratorjy spraydryer--Buchi/Brinkmann Mini Spray Dryer Model 190 of BrinkmannInstruments Co. The inlet temperature was 185° C. The spray dryer wasoperated so that the outlet temperatures were 80° to 90° C. During thespray dring operation, the chamber was brushed manually to simulate anair broom of a commercial production plant.

The spray dryability of the resin was determined by the powder resinyield based on the theoretical resin solids input to the spray dryer.The results of the spray dryability test are shown in Table 1:

                  TABLE 1                                                         ______________________________________                                        Resin No.    Powder Resin Yield                                               ______________________________________                                        Control      41.8%                                                            Resin A      88.8%                                                            Resin B      92.6%                                                            ______________________________________                                    

The results in Table 1 indicate that Resin A and Resin B, which wereboth treated with borax pentahydrate, show significantly higher powderresin yield than the control sample. During spray drying the controlresin, a substantial amount of the powder resin was deposited on thesurfaces of the spray dryer chamber and cyclone. The deposited powderresin was too soft and sticky to be brushed off during the spray drying.Therefore, the results demonstrate that the spray dryability of the lowadvanced phenol-formaldehyde resin (Control) can be enhanced by theborax treatments.

The powder resins were further compounded with drying agents--1% calciumhydroxide and 0.1% calcium silicate based on weight of the powder resin.

ANALYSES OF POWDER RESINS OF EXAMPLE 1

The powder resins compounded with drying agents were analyzed fornon-volatile content, bulk density, thermal flow property, molecularweights, and particle size distribution. For determining thenon-volatile content, a 0.5+/-0.05 grams powder resin sample was driedin a 125° C. oven for 105 minutes. The non-volatile is the percent ofthe dried resin based on the original sample weight. For determinationof thermal flow property, a 12.7 mm (0.5") diameter resin pellet wasmade by pressing 0.5+/-0.05 grams powder sample with a Parr pelletpress. The resin pellet was then pressed on a 150 +/-1° C. hot plateunder 6 Kg weight pressure for 3.0 minutes. The average radius of theflattened pellet was measured.

Molecular weight distributions of the powder resins were determined by ahigh pressure gel permeation chromatograph. The equipment is a modelALC/GPC-201 with 10⁴ A°, 10³ A°, 500A°, and 100A° μ-styragel columnscombination from Waters Associates, Inc.

Resin sample for the gel permeation chromatograph was prepared bydissolving the powder resin in tetrahydrofuran solvent at about 0.3%concentration, then acidifying to pH 4 to 5 with one normal sulfuricacid and dehydrating with anhydrous sodium sulfate. Further, the resinsolution was filtered using a sample clarification kit.

The gel permeation chromatograph was operated by the followingconditions:

    ______________________________________                                        Solvent            Tetrahydrofuran                                            Temperature        25° C.                                              Detector           Refractive Index                                           Flow Rate          1.0 ml/min.                                                ______________________________________                                    

The molecular weight of the resin was determined by using a calibrationfrom reference polystyrenes, bisphenol A and phenol. Weight average (Mw)and number average (Mn) molecular weights were determined from thechromatograms. All molecular weights set forth in this application weredetermined by this same method. For the particle size distributiondetermination of the powdered resins, a 30 gram resin sample was shakenthrough a Tyler sieve set comprised of 100,200, and 325 mesh for 15minutes. The powder resins retained on each sieve and pan weredetermined based on the total powder resin weight.

The results of the powder resin samples, Control, Resin A, and Resin B,are shown in Table 2:

                  TABLE 2                                                         ______________________________________                                        Powder Resin Property                                                                         Control   Resin A   Resin B                                   ______________________________________                                        Non-volatile Content, %                                                                       90.1      91.9      92.6                                      Bulk Density, g/cm.sup.3                                                                      0.453     0.449     0.412                                     Radial Thermal Flow, mm                                                                       25.3      22.3      22.0                                      Average Molecular Weight                                                      Weight Average (Mw)                                                                           1630      1506      1561                                      Number Average (Mn)                                                                           531       525       508                                       Dispersity (Mw/Mn)                                                                            3.1       2.9       3.1                                       Powder Paricle Size                                                           Distribution                                                                  >100 mesh (>150 μm)                                                                        3.5       0.8       0.6                                       <100 >200 mesh  11.0      3.2       3.7                                       (<150 >75 μm)                                                              <200 >325 mesh  13.8      6.7       5.7                                       (<75 >45 μm)                                                               <325 mesh (<45 μm)                                                                         71.7      89.3      90.0                                      ______________________________________                                    

The results in Table 2 show that the borax treated Resin A and Resin Bhad lower molecular weights than the Control sample, although theirradial thermal flows were lower than the Control. This indicated thatthe borax treated Resin A and Resin b reduced the resin advancementduring the spray drying process. Also, the particle sizes of Resin A andResin B were finer than the Control.

EXAMPLE 2

This example is to evaluate three powder resins--the Control, Resin A,and Resin B--for making homogeneous waferboards. The waferboards weretested for internal bond and 2-hour boiled (accelerated aging) modulusof rupture (MOR). The Canadian Standards Association (CSA) standardCAN3-0437.1-M85 was followed.

Laboratory size (254×254×11.1 mm) waferboards were made at conditionssimilar to conventional production at a waferboard mill. Commercialaspen wood wafer furnish which contained 3.5% moisture content was firstsprayed with 1.5% molten slack wax and then blended with 2.3% powderresin based on dry wood weight.

The wax and resin treated wood furnish was formed into a mat andprepressed in a cold press. In simulating the broad waferboardproduction conditions, the mats were subjected to a pre-cure test beforehot pressing. For the pre-cure test, a 3.2 mm thick stainless steel caulplate was preheated to 120° C., 130° C., 140° C., and 25° C. as thecontrol caul temperature. Then the prepressed mat was placed on thepreheated caul plate for 10 minutes prior to hot-pressing. To ensuregood contact between mat and the preheated caul plate, a caul platewhich gave a pressure of 4.5 g/cm² on the mat was placed on top of themat. After the pre-cure test, the 11.1 mm thick waferboard was then madeby hot pressing at 210° C. press temperature for 4.5 minutes with amaximum pressure of 35.2 Kg/cm² (500 psi). The target board density was0.65 g/cm³.

Four (4) boards with 4 different caul temperatures pre-treatments weremade from each powder resin. Six (6) internal bond samples and one2-hour boiled MOR sample were cut from each board and tested accordingto the CSA standard CAN3-0437.1-M85. The rate of face failure (%) wasdetermined from the ratio of the number of face failure (breaks) samplesto the total internal bond tested specimens. The rate of face failureindicates the pre-cure resistance of the powder resin. The results areshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                               2-hr.                                  Caul.      Board    Internal    Face   Boiled                                 Temp.      Density  Bond.sup.a  Failure.sup.b                                                                        MOR                                    C.         g/cm.sup.3                                                                             MPa*        %      MPa                                    ______________________________________                                                            Resin: Control                                             25        0.65     0.459       0      14.8                                   120        0.65     0.472       0      10.7                                   130        0.65     0.415       33     10.7                                   140        0.65     0.420       33     10.7                                   Average    0.65     0.442              11.7                                                       Resin A                                                    25        0.67     0.506       0      12.8                                   120        0.64     0.462       0      12.6                                   130        0.67     0.443       17     14.6                                   140        0.64     0.537       0      11.2                                   Average    0.66     0.487              12.8                                                       Resin B                                                    25        0.66     0.508       0      16.9                                   120        0.67     0.523       0      17.0                                   130        0.68     0.456       0      13.0                                   140        0.65     0.456       17     14.0                                   Average    0.67     0.486              15.3                                   CAN3-0437.0-M85     0.345              8.6                                    Requirements                                                                  ______________________________________                                         .sup.a Average of six specimens.                                              .sup.b Ratio of face failure specimens to the total internal bond tested      specimens.                                                                    *Mega Pascals                                                            

The results shown in Table 3 indicate that the borax modified resins,Resin A and Resin B, as well as the Control Powder Resin, were pre-cureresistant for waferboard manufacture, and the resins satisfied the widerange of caul plate temperatures present in commercial production lines.The properties of waferboards made with the powder resins passed the CSAstandard.

EXAMPLE 3

The liquid phenol-formaldehyde resin made in Example 1 was furthertreated with different amounts of borax pentahydrate and tested forspray dryability.

    ______________________________________                                                         Parts By Weight                                              Ingredient         Resin C  Resin D                                           ______________________________________                                        Borax Pentahydrate  0.1      0.4                                              (Na.sub.2 B.sub.4 O.sub.7.5H.sub.2 O)                                         Water              36.2     36.1                                              Liquid Resin (47.0% solids)                                                                      63.7     63.5                                              ______________________________________                                    

The borax pentahydrate was first dissolved in the water and mixed withthe liquid resin. The borax modified liquid resins had the followingproperties:

    ______________________________________                                                           Resin C  Resin D                                           ______________________________________                                        Resin Solids Content, %                                                                            30.0       30.3                                          Refractive Index (25° C.)                                                                   1.4137     1.4141                                        Viscosity (25° C.) (Gardner-Holdt)                                                          A1A1-A     A                                             ______________________________________                                    

Resin C and Resin D were spray dried as described in Example 1. Thepower resin yields are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Resin       Powder Resin Yield                                                ______________________________________                                        Resin C     74.2%                                                             Resin D     84.5%                                                             ______________________________________                                    

Without borax treatment, the powder resin yield was only 41.8% (Example1); therefore, the results shown in Table 4 indicate that the boraxtreatments (Resin C and Resin D) significantly improved the powder resinyields.

Powder Resin C and Resin D were further compounded with 1% calciumhydroxide and 0.1% calcium silicate based on the weight of powder resin.The powder resins were then tested for non-volatile content, bulkdensity, thermal flow property, and particle size distribution. Theresults are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Powder Resin Property                                                                            Resin C  Resin D                                           ______________________________________                                        Non-volatile Content, %                                                                          91.3     91.5                                              Bulk Density, g/cm.sup.3                                                                          0.38    --                                                Radial Thermal Flow, mm                                                                          23.0     22.0                                              Particle Size Distribution                                                    >100 mesh (>150 μm)                                                                           1.0       1.0                                              <100 >200 mesh     5.7       3.5                                              (<150 >75 μm)                                                              <200 >325 mesh     8.3       8.2                                              (<75 >45 μm)                                                               <325 mesh (<45 μm)                                                                            85.0     87.3                                              ______________________________________                                    

EXAMPLE 4

This example showed that boric acid (H₃ BO₃) improved spray dryabilityof a phenol-formaldehyde resin. The liquid phenolic resin made inExample 1 was treated with different amounts of boric acid as follows:

    ______________________________________                                                         Parts by Weight                                              Ingredient         Resin E  Resin F                                           ______________________________________                                        Boric Acid (H.sub.3 BO.sub.3)                                                                     0.1      0.3                                              Water              36.2     36.4                                              Liquid Resin (47.0% solids)                                                                      63.7     63.3                                              ______________________________________                                    

The boric acid was dissolved in the water, then mixed with the liquidphenol-formaldehyde resin. The properties of the boric acid treatedliquid resin were as follows:

    ______________________________________                                                           Resin E Resin F                                            ______________________________________                                        Resin Solids Content, %                                                                            30.0      30.1                                           Refractive Index (25° C.)                                                                   1.4144    1.4145                                         Viscosity (25° C.)(Gardner-Holdt)                                                           A1A1-A    A                                              ______________________________________                                    

The resins were spray dried as described in Example 1, and the powderresin yields are shown in Table 6:

                  TABLE 6                                                         ______________________________________                                        Resin       Powder Resin Yield                                                ______________________________________                                        Resin E     74.5%                                                             Resin F     91.0%                                                             ______________________________________                                    

A powder resin yield from spray drying a control sample, without boricacid treatment, was only 41.8%. Therefore, the results shown in Table 6indicate that the boric acid treatments significantly improved spraydryability of the liquid resin.

The spray dried powder resins were further compounded with 1% calciumhydroxide and 0.1% calcium silicate, based on weight of the powderresin, and then tested for non-volatile content, bulk density, thermalflow property, and particle size distribution. The results are shown inTable 7:

                  TABLE 7                                                         ______________________________________                                        Powder Resin Properties                                                                            Resin E   Resin F                                        ______________________________________                                        Non-volatile Content, %                                                                            92.0      92.6                                           Bulk Density, g/cm.sup.3                                                                           0.41      0.38                                           Radial Thermal Flow, mm                                                                            23.8      21.8                                           Particle Size Distribution                                                    >100 mesh >150 μm)                                                                              1.7       1.0                                            <100 >200 mesh (<150 >75 μm)                                                                    7.3       4.0                                            <200 >325 mesh (<75 >45 μm)                                                                     10.0      5.7                                            <325 mesh (<45 μm)                                                                              81.0      89.3                                           ______________________________________                                    

EXAMPLE 5

A low advanced phenol-formaldehyde liquid resin was prepared, furthertreated with sodium borate, and tested for spray dryability.

A resin reactor was charged with the following ingredients:

    ______________________________________                                                              Parts by Molar                                          Ingredients           Weight   Ratio                                          ______________________________________                                        Phenol (92% conc.)    33.93    1                                              First Formaldehyde (46.5% conc.)                                                                    32.14    1.5                                            First Water           10.51                                                   Sodium Hydroxide (50% conc.)                                                                        3.18     0.12                                                                 (79.76)                                                 Second Formaldehyde (46.5% conc.)                                                                   12.85    0.6                                            Second Water          2.76                                                    Third Formaldehyde (46.5% conc.)                                                                    2.11     0.1                                            Ammonium Hydroxide (28% conc.)                                                                      2.20                                                    Oleic Acid            0.32                                                                          (20.24)                                                 ______________________________________                                    

The reactor was charged with phenol, first formaldehyde, and firstwater. Then the sodium hydroxide was added slowly over a 10 to 15 minuteperiod, and the temperature was allowed to rise to 95° C. in 50 minutes.The temperature was held at 95° C. until E to F of the Gardner-Holdtviscosity (25° C.), then cooled to 75° to 70° C. The second formaldehydeand second water were added to the resin, and the temperature wasadjusted to 70° C. The reaction was continued at 70° C. until theGardner-Holdt viscosity (25° C.) was G to H. The temperature was reducedto below 55° C., then the third formaldehyde, ammonium hydroxidesolution, and oleic acid were added. Cooling was continued to below 25°C.

The properties of the liquid resin are as follows:

    ______________________________________                                        Viscosity (25° C.)                                                                         J-KK                                                      Solids Content      47.0%                                                     Refractive Index (25° C.)                                                                  1.4675                                                    Water Dilutability (25° C.)                                                                167%                                                      ______________________________________                                    

The liquid resin was treated with different amounts of boraxpentahydrate as follows:

    ______________________________________                                                   Parts By Weight                                                    Ingredient   Control  Resin G  Resin H                                                                              Resin I                                 ______________________________________                                        Borax pentahydrate                                                                         0        0.2      0.4    1.0                                     (Na.sub.2 B.sub.4 O.sub.7.5H.sub.2 O)                                         Water        36.2     36.1     34.2   35.8                                    Liquid Resin 63.8     63.7     65.4   63.2                                    (47.0% solids)                                                                ______________________________________                                    

The borax pentahydrate was first dissolved in the water and then mixedwith the resin. The Control sample had no borax treatment. Properties ofthe Control and the borax treated resins are shown as follows:

    ______________________________________                                                            Resin   Resin   Resin                                                  Control                                                                              G       H       I                                         ______________________________________                                        Resin Solids Content, %                                                                      30.0     30.1    30.2  30.7                                    Refractive Index (25° C.)                                                             1.4135   1.4132  1.4161                                                                              1.4140                                  Viscosity (25° C.)                                                                    A1A1-A   A1A     AB    EF                                      ______________________________________                                    

The liquid resins were spray dried as described in Example 1. The powderresin yields are shown in Table 8:

                  TABLE 8                                                         ______________________________________                                                   Control                                                                              Resin G  Resin H  Resin I                                   ______________________________________                                        Powder Resin Yield, %                                                                      43.0     74.5     88.0   91.7                                    ______________________________________                                    

Again, the results in Table 8 indicate that the borax treated resins,Resin G, Resin H, and Resin I, significantly enhance the powder resinyield from the Control sample.

The powder resins were further compounded with 1% calcium hydroxide and0.1% calcium silicate based on the weight of the powder resin and testedfor non-volatile content, bulk density, thermal flow property, averagemolecular weight, and particle size distribution. The results are shownin Table 9:

                  TABLE 9                                                         ______________________________________                                                                 Resin   Resin Resin                                  Powder Resin Property                                                                         Control  G       H     I                                      ______________________________________                                        Non-volatile Content, %                                                                       91.3     92.2    91.7  91.5                                   Bulk Density, g/cm.sup.3                                                                      --       0.40    0.44  0.47                                   Radial Thermal Flow, mm                                                                       26.5     24.5    22.5  18.5                                   Average Molecular Weight                                                      Weight Average (Mw)                                                                           1458                   1323                                   Number Average (Mn)                                                                           507                    456                                    Dispersity (Mw/Mn)                                                                            2.9                    2.9                                    Particle Size Distribution                                                    >100 mesh (>150 μm)                                                                        13.9     1.7     1.5   1.7                                    <100 >200 mesh  21.7     8.0     4.8   7.2                                    (<150 >75 μm)                                                              <200 >325 mesh  25.2     13.3    7.0   10.8                                   (<75 >45 μm)                                                               <325 mesh (<45 μm)                                                                         39.2     77.0    86.7  80.3                                   ______________________________________                                    

The results in Table 9 indicate that the borax treated resins, Resin G,Resin H, and Resin I, had lower radial thermal flow and smaller particlesize than the Control powder resin. Although the radial thermal flow ofResin I is substantially smaller than the Control, the average molecularweight of the borax treated Resin I is lower than the Control. Again,this demonstrated that the borax treatment protected the Resin fromadvancement during the spray drying process.

EXAMPLE 6

This example demonstrates that spray dryability of the liquidphenol-formaldehyde resin prepared in Example 5 was improved by boricacid treatments.

The resins treated with differnt amounts of boric acid are shown asfollows:

    ______________________________________                                                        Parts By Weight                                               Ingredient        Resin J Resin K                                             ______________________________________                                        Boric Acid (H.sub.3 BO.sub.3)                                                                   0.3     0.6                                                 Water             36.4    36.3                                                Resin (47.0% solids)                                                                            63.3    63.1                                                ______________________________________                                    

The boric acid was first dissolved in the water, then mixed with thephenol-formaldehyde resin. The properties of the boric acid treatedresin are as follows:

    ______________________________________                                                          Resin J                                                                             Resin K                                               ______________________________________                                        Resin Solids Content, %                                                                           30.1    30.3                                              Refractive Index (25° C.)                                                                  1.4128  1.4131                                            Viscosity (25° C.)                                                                         A       DD-E                                              ______________________________________                                    

The resins were spray dried as described in Example 1. The powder resinyields based on the resin solids input to the spray dryer are shown inTable 10.

                  TABLE 10                                                        ______________________________________                                        Resin No.          Resin J Resin K                                            ______________________________________                                        Powder Resin Yield, %                                                                            66.3    75.3                                               ______________________________________                                    

As shown in Table 10, the powder resin yields from the boric acidtreated resins, Resin J and Resin K, improved from the Control sampleshown in Example 5. The Control sample, without boric acid treatment,had a powder resin yield of only 43%.

The powder resins were further compounded with 1% calcium hydroxide and0.1% calcium silicate based on the weight of the powder resin; theresins were then tested for non-volatile content, bulk density, thermalflow property, and particle size distribution. The results are shown inTable 11.

                  TABLE 11                                                        ______________________________________                                        Powder Resin Property Resin J Resin K                                         ______________________________________                                        Non-Volatile Content, %                                                                             92.5    91.5                                            Bulk Density, g/cm.sup.3                                                                            0.352   0.407                                           Radial Thermal Flow, mm                                                                             23.0    23.5                                            Particle Size Distribution                                                    >100 mesh (>150 μm)                                                                              3.0     2.3                                             <100 >200 mesh (<150 >75 μm)                                                                     10.2    10.2                                            <200 >325 (<75 >45 μm)                                                                           15.2    11.2                                            <325 mesh (<45 μm) 71.6    76.3                                            ______________________________________                                    

EXAMPLE 7

The example is to test powder Resin G and Resin I from Example 5 andResin K from Example 6 for waferboard manufacture. The waferboards weretested for internal bond and 2-hour boiled (accelerated aging) modulusof rupture (MOR). Canadian Standard CAN3-0437.1-M85 was followed.

Laboratory size (254×254×11.1 mm) waferboards were made at conditionssimilar to a conventional waferboard production line. Commercial aspenwood wafer furnish of 4.5% moisture content was first sprayed with 1.5%molten slack wax and then blended with 2.3% powder resin based on thedry wood weight.

The wax and powder resin treated wood furnish was formed into a mat andprepressed in a cold press. To simulate wide waferboard manufacturingconditions, the mat was subjected to a pre-cure test before hotpressing. For the pre-curing test, 3.2 mm thick stainless steel caulplates were pre-heated to the temperatures of 120° C., 130° C., and 140°C. A control caul temperature was 25° C. The prepared mats were placedon the preheated caul plate for 10 minutes prior to hot pressing. Toensure good contact between mat and the hot caul plate, a caul platewhich gave a pressure of 4.5 g/cm² on mat was placed on top of the mat.Then the 11.1 mm thick waferboard was made by hot pressing at 210° C.press temperature for 4.5 minutes with a maximum pressure of 35.2 Kg/cm²(500 psi). The target board density was 0.65 g/cm³.

Four (4) boards with 4 caul temperatures of pre-cure test were made fromeach powder resin. Six (6) internal bond samples and one 2-hour boiledMOR sample were prepared from each panel and tested according toCAN3-0437.1-M85. The rate of face failure (%) was determined from theratio of the number of face failure (breaks) samples to the totalinternal bond tested specimens. The rate of face failure indicatespre-cure resistance of the powder resin. The results are shown in Table12.

                  TABLE 12                                                        ______________________________________                                                                             2-hour                                   Caul      Board    Internal   Face   Boiled                                   Temp.     Density  Bond.sup.a Failure.sup.b                                                                        MOR                                      °C.                                                                              g/cm.sup.3                                                                             MPa*       %      MPa                                      ______________________________________                                        Resin G (from Example 5)                                                      25        0.67     0.556      0      15.2                                     120       0.75     0.552      0      17.4                                     130       0.74     0.512      17     19.6                                     140       0.66     0.491      0      14.6                                     Average   0.69     0.533             15.7                                     Resin I (from Example 5)                                                      25        0.66     0.438      0      13.9                                     120       0.69     0.570      0      17.0                                     130       0.74     0.512      17     19.6                                     140       0.68     0.493      17     11.8                                     Average   0.69     0.503             15.6                                     Resin K (from Example 6)                                                      25        0.70     0.469      0      14.6                                     120       0.70     0.498      0      19.5                                     130       0.73     0.502      0      13.9                                     140       0.69     0.473      33     16.2                                     Average   0.71     0.486             16.1                                     CAN3-0437.0-M85                                                                              0.345              8.6                                         Requirements                                                                  ______________________________________                                         .sup.a Average of six specimens                                               .sup.b Ratio of face failure specimen to the total internal bond tested       specimen.                                                                     *Mega Pascals                                                            

The results in Table 12 indicate that borax treated Resin G and Resin I,as well as boric acid treated Resin K, were pre-cure resistant forwaferboard manufacture, and the powder resins satisfied the wide rangeof caul plate temperatures present in commercial production lines.Morever, the waferboard properties made with the powder resins surpassedthe CSA standard.

What I claim is:
 1. A method for preparing a curable phenol-aldehydespray dried powder which comprises spray drying an aqueousphenol-aldehyde resole resin containing from about 0.05% to 5% by weightof a water soluble oxo boron compound, the quantity of said boroncompound being on an anhydrous basis in relation to the resin solids ofsaid resole.
 2. The method of claim 1 wherein the aldehyde isformaldehyde, acetaldehyde, paraformaldehyde, or propionaldehyde.
 3. Themethod of claim 1 wherein 35 to 45% by weight of the resin has a weightaverage molecular weight of about 120 to
 520. 4. The method of claim 1wherein the boron compound is selected from the group consisting ofboric acid, sodium borates, potassium borates, lithium borates, ammoniumborates, metaboric acid, sodium metaborates, potassium metaborates,lithium metaborates, and ammonium metaborates.
 5. A method for preparinga curable phenol-formaldehyde spray dried powder which comprises spraydrying an aqueous solution of a phenol-formaldehyde resole having aresin solids content of from about 25% to 45% by weight of the aqueoussolution and a viscosity of about 30 to 300 Cps at 25° C., said resolecontaining from about 0.05% to 5% by weight of a water soluble oxo boroncompound, the quantity of said boron compound being on an anhydrousbasis in relation to the resin solids of said resole.
 6. A method forpreparing a curable phenol-formaldehyde spray dried powder whichcomprises spray drying at a spray dried inlet temperature of about 180°C. to 210° C. and an outlet temperature of about 60° C. to 95° C., anaqueous solution of a phenol-formaldehyde resole resin, said resinprepared by reacting about 1.5 to 2.5 moles of formaldehyde for eachmole of phenol in the presence of about 0.05 to 0.25 moles of analkaline catalyst to prepare an initial aqueous solution of aphenol-formaldehyde resole resin, admixing into said initial solutionfrom about 0.05% to about 5% by weight of a water soluble oxo boroncompound, the quantity of said boron compound being on an anhydrousbasis in relation to the resin solids and adding additional water to theinitial solution to form a less concentrated solution suitable for spraydying.
 7. The method of claim 6 wherein the quantity of the boroncompound varies from about 0.1% to 2%.
 8. The method of claim 6 whereinthe oxo boron compound is selected from the group consisting of boricacid, sodium borates, potassium borates, lithium borates, ammoniumborates, metaboric acid, sodium metaborates, potassium metaborates,lithium metaborates, and ammonium metaborates.
 9. The method of claim 6wherein the resin is made in a two-stage reaction wherein for the firststage the molar ratios of phenol to formaldehyde to alkali catalyst are1:(1.4-1.6):(0.1-0.15), and the reaction temperature varies from about90° C. to reflux and formaldehyde is then added thereto so that at theend of the second stage the molar ratios of phenol to formaldehyde toalkali catalyst are 1:(1.8-2.2):(0.1-0.15) and wherein the reactiontemperature during the second stage is from about 55° C. to 70° C. 10.The method of claim 6 wherein the aqueous resole resin solution containsfrom about 40% to about 55% by weight of resin solids and the viscosityvaries from about 200 Cps to 900 Cps at 25° C. prior to admixing of theboron compound and addition of water for the spray drying step.
 11. Themethod of claim 6 wherein 35% to 45% by weight of the resin solids has aweight average molecular weight of about 120 to
 520. 12. A method forimproving the spray drying of an aqueous phenol-formaldehyde resoleresin solution wherein said resin has a weight average molecular weightof from about 1200 to 1800 to form a curable spray dried resin whichcomprises spray dyring said aqueous resin solution having dissolvedtherein from about 0.1 percent to 2 per cent of a water soluble oxoboron compound, the quantity of said boron compound being on ananhydrous basis in relation to the resin solids of said resole.